The present invention relates to a smear removing method suitable for removing smear from via holes, which is performed before plating copper in the via holes for interconnecting wiring layers in a multi-layer substrate of a semiconductor.
To increase the density of a multi-layer wiring board of a semiconductor, build-up wiring boards are used. The build-up wiring board employs via holes for interconnecting wiring layers. If the via holes are used only for interconnecting two layers, the inner wall of the via holes need not be plated. However, when three or more layers must be interconnected, via holes 31 not filled with plated metal must be displaced as shown in FIG. 4(a). On the other hand, via holes 31 filled with plated metal can be stacked as shown in FIG. 4(b), allowing a higher degree of freedom in the layout of wires, and a higher mounting density.
Conventional via holes have diameters of approximately 100 xcexcm, so that via holes not plated inside do not impede the conductance between each of the layers. However, as via holes have smaller diameters, via holes not plated inside cause an increase in resistance to impede the conductance between each of the layers.
Via holes are formed by photo-etching using a photoresist or by irradiating a laser using a resin other than a photoresist as an insulating layer.
After via holes are formed, a smear removing process, a catalyst applying process, a chemical copper plating process, and an electrolytic copper plating process are performed before the plating is completed. As shown in FIG. 5(a), the smear removing process removes a resin remaining on the bottom of a via hole 34 (hereinafter called the xe2x80x9csmearxe2x80x9d) when the via hole 34 is formed through an insulating layer 33 which overlies a lower wiring layer 32 formed on a substrate (not shown).
Then, conventionally, the smear removing process includes the five steps of swelling, permanganic acid etching, neutralization/reduction, cleaner conditioning, and soft etching, which are performed sequentially.
When the via hole 31 has a diameter of 100 xcexcm, the insulating layer 33 is formed with the via hole 34 by a photolithography method, which uses a photoresist. If the via hole 31 has a diameter of smaller than approximately 40 xcexcm, the photolithography method encounters difficulties in forming the clear via hole 34. For this reason, the via hole 34 is formed by irradiation of laser. However, when the via holes 34 formed by the irradiation of laser undergo the smear removing process under similar conditions to those when the via holes 34 are formed by the photolithography method, most of the via holes 34 do not satisfy a thermal shock test, which is one item of a via hole reliability test conducted after filling the via holes 31 with plated metal. The thermal shock test involves 1000 repetitions of immersion in a liquid at xe2x88x9255xe2x96xa1C and in a liquid at 125xc2x0 C. each for three minutes, and a resistance changing percentage within xc2x110% is determined as passed.
A soft etchant used in the soft etching step, which is the last processing in the conventional smear removing process, serves to slightly roughen the surface of a copper foil which comprises the lower wiring layer 32 for closing lower ends of via holes, as well as serves to remove smear. Conventionally, used as the soft etchant is a sulfuric acid solution (sulfuric acid concentration: 98 g/l), a sulfuric acid and sodium persulfate solution (sulfuric acid concentration: 9.8 g/l, sodium persulfate concentration: 150 g/l), or a sulfuric acid and hydrogen peroxide solution (sulfuric acid concentration 98 g/l, hydrogen peroxide concentration: 27 g/l).
Conventionally, since the via hole 34 is formed by the photolithography method, only a small amount of smear is produced, so that the soft etchant relatively satisfactorily removes smear and roughens the copper foil. However, the use of laser irradiation for forming the via hole 34 causes an increased amount of smear. Therefore, when the same soft etchant is used for the soft etching under the same conditions as in the conventional process, the smear is not completely removed, with the result that the via hole is not satisfactorily filled with plated metal. Also, when the dissolving power of the soft etchant is excessively increased, the lower wiring layer 32 is excessively etched to produce a gap A between the copper foil and resin, as shown in FIG. 5(b). If the via hole is plated in the shown state, a portion corresponding to the gap is more likely to crack in the thermal shock test, which is conducted after the via hole is filled with plated metal.
It is an objective of the present invention to provide a method of removing smear from via holes, which is capable of removing the smear without fail in a smear removing process without excessively etching a conductive layer when the via holes have small diameters, for example, even when via holes having a diameter of 40 xcexcm are formed by the irradiation of laser.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a method of removing smear from via holes is provided. The method includes: providing a multi-layer substrate that has a conductive layer and an insulating layer provided on the conductive layer; forming a via hole in the insulating layer provided on the conductive layer; removing smear remaining on a part of the conductive layer that is the bottom of the via hole by soft etching with a soft etchant, wherein the soft etchant is an aqueous solution containing a sulfuric acid and hydrogen peroxide, and wherein the concentration of the sulfuric acid is 2.4 times the concentration of hydrogen peroxide or less; forming a chemical copper plating layer on the entire inner surface of the via hole including the bottom by performing chemical copper plating; and forming fill-plating layer in the via hole by performing electrolytic copper plating.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.